Mice with segmental trisomy 16 (Ts65Dn mice) are at gene dosage imbalance for many of the same genes triplicated in human beings with Down Syndrome (DS), the leading genetic cause of mental retardation in human beings. Marked changes in brain volume and development are notable in DS. We have recently shown that Ts65Dn mice have a marked deficit in learning and memory tasks. We wish to compare the structure of affected mouse brains to controls, initially in adults and subsequently during development. Results will be compared to ongoing MRI studies of DS individuals at Johns Hopkins.